Conventionally a traveling driving device for a bicycle has a mechanism, in which a chain wheel is rotated by pedaling action through a gear crank and rotation of the chain wheel is transmitted to a rear wheel through a chain.
In the traveling driving device constructed as described above, it is one of the simplest ways to elongate the arm of the gear crank in order to intend increase of torque of the rotation by the pedaling action. However, since the most appropriate length of the arm of the gear crank is determined from points of view of the human engineering and the construction of the driving mechanism, if it is too long, the pedaling action is difficult so that it is not practicable.